Quantum Electrodynamics (QED)

In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism giving a complete account of matter and light interaction.

https://www.nobelprize.org/prizes/physics/1965/feynman/lecture/

https://www.nobelprize.org/prizes/physics/1965/ceremony-speech/

https://www.quantamagazine.org/physicists-measure-the-magic-fine-structure-constant-20201202/
The fine-structure constant, by contrast, has no dimensions or units. It’s a pure number that shapes the universe to an astonishing degree — “a magic number that comes to us with no understanding,” as Richard Feynman described it. Paul Dirac considered the origin of the number “the most fundamental unsolved problem of physics.”